In a gas turbine engine, ambient air is drawn into a compressor section. Alternate rows of stationary and rotating aerofoil blades are arranged around a common axis, together these accelerate and compress the incoming air. A rotating shaft drives the rotating blades. Compressed air is delivered to a combustor section where it is mixed with fuel and ignited. Ignition causes rapid expansion of the fuel/air mix which is directed in part to propel a body carrying the engine and in another part to drive rotation of a series of turbines arranged downstream of the combustor. The turbines share rotor shafts in common with the rotating blades of the compressor and work, through the shaft, to drive rotation of the compressor blades.
It is well known that the operating efficiency of a gas turbine engine is improved by increasing the operating temperature. The ability to optimise efficiency through increased temperatures is restricted by changes in behaviour of materials used in the engine components at elevated temperatures which, amongst other things, can impact upon the mechanical strength of the blades and rotor disc which carries the blades. This problem is addressed by providing a flow of coolant through and/or over the turbine rotor disc and blades.
It is known to take off a portion of the air output from the compressor (which is not subjected to ignition in the combustor and so is relatively cooler) and feed this to surfaces in the turbine section which are likely to suffer damage from excessive heat. Typically the cooling air is delivered adjacent the rim of the turbine disc and directed to a port which enters the turbine blade body and is distributed through the blade, typically by means of a labyrinth of channels (not shown) extending through the blade body.
In one known arrangement, a duct is provided integral to the blade. The duct is arranged to pass through a terminal portion of the root with an inlet at an upstream face of the terminal portion and an end at or near the downstream face of the terminal portion. At its axially upstream face, the terminal portion is profiled to conform closely to the bucket groove profile and an inner wall defines the inlet which has a similar shape to the terminal portion at the upstream face. At its axially downstream end, the duct has a much smaller cross section, radially distanced from the bucket groove. An inner wall of the duct diverges from the downstream end of the duct to the inlet forming a continuous, inclined passage. One or more cooling passages are provided within the blade body and extend from a root portion towards a tip portion of the blade body.
In some arrangements the cooling passages comprise a leading edge passage and a main blade or “multi-pass” passage. The leading edge passage extends root to tip adjacent the leading edge of the blade. The “multi-pass” passage is an elongate and convoluted passage which typically incorporates multiple turns in three dimensions which extend the passage between the root and tip of the blade and from a middle section of the blade body, downstream to adjacent the trailing edge of the blade. The “multi-pass” can extend from root to tip multiple times as it travels towards the trailing edge ensuring the carriage of coolant throughout the blade body (excluding the leading edge which is cooled by the leading edge passage.). At the root portion end, the cooling passages are arranged to intersect with the duct.